1. Field of the Invention
The present invention relates to a method and an apparatus for gasifying and combusting a liquid fuel.
2. Description of the Prior Art
The combustion modes of kerosene are classified generally into a blue flame combustion mode and a white flame combustion mode. A premixed flame containing sufficient oxygen is a blue flame and a diffusive flame is a white flame (luminous flame). The kerosene combusting process is supposed to be as represented by the flow chart of FIG. 4. As is apparent from FIG. 4, the mode of combustion is dependent on the mode of diffusion of oxygen. Factors that affect the diffusion of oxygen are:
1. quantity of air (oxygen) supplied for combustion, PA1 2. mass of the fuel, and PA1 3. the turbulence of the flow of air-fuel mixture.
Blue flame combustion requires sufficient air for combustion, well atomized fuel particles of small mass (complete atomization of the fuel into fine droplets), or a turbulent flow of the air-fuel mixture for completely mixing the fuel and air.
With the conventional gun type burner which is widely used for combusting kerosene, it is difficult to achieve blue flame combustion even if the above-mentioned requirements of blue flame combustion are met. Such difficulty is attributable to the construction of the conventional gun type burner and the resultant mode of combustion. In the conventional gun type burner, a flame holding member is disposed right behind a fuel spraying nozzle, and thereby a stable flame which serves as an ignition source for igniting the successively sprayed fuel is formed in the flame holding member. Therefore, the actions in all the stages of the combustion process shown in FIG. 4 occur simultaneously, and hence each fuel droplet is ignited over the surface thereof before it is gasified, and each fuel droplet then burns in a diffusive flame ball consisting of a central core of the liquid fuel and a flame shell concentrically surrounding the central core of the liquid fuel. This mode of combustion causes unsatisfactory diffusion of oxygen and results in white flame combustion instead of blue flame combustion.
In white flame combustion (diffusive combustion), oxygen needs to diffuse into the fuel droplet through the flame of the diffusive flame ball and mixing the fuel and air needs to be accelerated by the turbulent flow of air or by supplying excessive air. Accordingly, a white flame burner needs to be equipped with means to generate a turbulent air flow (for example, a maximum impulse flow) and means to supply excessive air.
In the white flame combustion, carbon is oxidized in the colloidal state, and, if oxygen is not diffused satisfactorily, carbon is discharged in the form of soot. Such unsatisfactory diffusion of oxygen is possible due to local irregular mixing of the fuel and air even if sufficient oxygen is supplied.
Furthermore, unsatisfactory diffusion of oxygen causes the production of intermediate gaseous products of oxidation (in most cases, carbon monoxide).
The amount of those products of combustion contained in the exhaust gas increases remarkably when the excessive air ratio is reduced. Accordingly, in the diffusive combustion, it has been difficult to reduce the excess air ratio below a certain level.
In the white flame combustion, it is necessary to generate a turbulent flow to accelerate air-fuel mixing. However, the turbulent flow includes diffusive flame balls which are burning and generates loud noises. Thus, the turbulent flow has been the principal causes of combustion noise. It has also been a problem in the white flame combustion that, if the magnitude of the turbulence of the flow is enhanced to achieve complete combustion, the combustion noise increases proportionally. Furthermore, it is usual to form a narrow passage behind the flame holding member to generate the turbulent flow, which raises the level of combustion noise still further because the flame is formed in a narrow space having an opening.
In the blue flame combustion, since oxygen diffuses easily into the evaporated gasiform fuel, an intensive turbulent flow and excessive air are not necessary.
Accordingly, in the blue flame combustion, the excess air ratio can be reduced near to the stoichiometric mixture ratio.
In the blue flame, a smaller amount of colloidal carbon is produced, the gasiform fuel and oxygen are diffused and mixed satisfactorily, the production of soot and carbon monoxide is reduced, and nearly complete combustion is achieved even if the excess air ratio is small.
Accordingly, in the blue flame combustion, the excess air ratio can be reduced near to the stoichiometric mixture ratio, and the flame temperature rises near to the adiabatic flame temperature since the fuel and air are mixed well and the combustion zone is narrowed.
Furthermore, since the blue flame combustion occurs after the fuel and air have been completely mixed and the flame is formed near the open end of the flame holding member, only low combustion noise is generated.
It is obvious from what has been described hereinbefore that the blue flame combustion is superior to the white flame combustion in burning a liquid fuel in gas phase.
Few improvements in the combustion process employing a liquid fuel gasification burner of this kind have been made on the basis of the principle of combustion so far, and the most of those improvements have been partial improvements in the components of the burner.
The prior arts disclosed in Japanese Patent Publication No. Sho 39-21913, Japanese Utility Model Publication No. Sho 57-32341 and Japanese Patent Laid-open No. Sho 55-41393, which are considered to be closely connected with the present invention, employed a porous ceramic burner cone in the combustion zone. However, those prior arts are not based on the fundamental principle of combustion, and the effective use of heat, such as the circulation of the combustion gas, is not taken into consideration. According to the invention disclosed in Japanese Patent Laid-open No. Sho 55-41393, since a flame holding member is disposed right in front of a nozzle, fuel droplets are ignited in the flame holding member before they are gasified completely, and hence perfect blue flame combustion is impossible.
Japanese Patent Laid-open No. Sho 58-200911 discloses a gun type burner equipped with a porous or net-form flame holding plate and adapted to utilize combustion gas circulating process. However, since the combustion gas flowing in a circumferential direction along the wall of the furnace is circulated, the gasification of the fuel in the initial stage of combustion is delayed, which lowers the ignitability of the fuel. Furthermore, since the jet of the air-fuel mixture is directed at the flame holding plate, the fuel droplets are ignited upon their collision against the flame holding plate and start burning before they are gasified completely. Accordingly, white flame combustion occurs, and perfect blue flame combustion is impossible, which reduces the thermal efficiency.